Follower vehicle control system and method for forward and reverse convoy movement

ABSTRACT

A vehicle control system for causing a follower vehicle to follow a leader may have a tether system mounted to the follower vehicle. The tether system may include a tether having an end adapted to be attached to the leader, a length sensor, and an angle sensor. A path tracking system operatively associated with the tether system determines a path traveled by the leader. A path control system operatively associated with the path tracking system and the follower vehicle causes the follower vehicle to follow the path traveled by the leader. A spacing control system operatively associated with the path tracking system and the follower vehicle causes the follower vehicle to maintain a predetermined spacing between the follower vehicle and the leader.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of co-pending U.S. applicationSer. No. 12/238,733, filed Sep. 26, 2008, which claims the benefit ofU.S. Provisional Application No. 61/189,527, filed Aug. 20, 2008, andU.S. Provisional Application No. 61/091,273, filed Aug. 22, 2008, all ofwhich are incorporated herein by reference for all that they disclose.

TECHNICAL FIELD

The invention relates to vehicle control systems in general and morespecifically to control systems for vehicle convoys.

BACKGROUND

A convoy can be used to transport large quantities of items to a certaindestination in order to reduce the amount of time and number of returntrips required, provide support for vehicles in order to attain safetravel, and can also be used to reduce traffic congestion on roadways. Arobotic convoy can be used to further the efficiency of convoy missionsin many different areas.

The robotic convoy concept can be used with many different types ofrobotic vehicles. In its simplest form, the robotic convoy includes aleader vehicle and a follower vehicle. The leader and follower vehiclescan be in two-way communication throughout a mission. It is thenpossible for the follower vehicle to receive information from the leadervehicle in order to aid the follower vehicle in following the path ofthe leader vehicle.

In the past, different methods for following the leader vehicle havebeen developed. One such method uses the global positioning system (GPS)to provide position information for the various vehicles. Unfortunately,however, GPS receives require a direct view or “line-of-sight” access toat least three, and typically more, GPS satellites. This direct view orline-of-sight limitation typically prevents GPS systems from being usedaround large buildings (e.g., in urban areas), in tunnels, orunderground. Furthermore, the position update frequency of most GPSsystems is not rapid enough to allow for fast traveling where the convoyvehicles are close and rapid response times are required.

Another type of convoy system uses magnetic markers wherein permanent,discrete magnetic markers line the path to be driven. This system haslimited utility in that it can only be used where magnet markers arepresent. Camera-based robotic convoy systems have also been developed inwhich the follower vehicle uses one or more cameras to track the leadervehicle. However, the data received from the camera is too large andcumbersome for use at high speeds. Another drawback is that the leadervehicle must always be in the line of sight of the camera.

SUMMARY OF THE INVENTION

One embodiment of a vehicle control system for causing a followervehicle to follow a leader may comprise a tether system mounted to thefollower vehicle, the tether system including a tether having an endadapted to be attached to the leader, a length sensor operativelyassociated with the tether, and an angle sensor operatively associatedwith the tether. A path tracking system operatively associated with thetether system determines a path traveled by the leader. A path controlsystem operatively associated with the path tracking system and thefollower vehicle causes the follower vehicle to follow the path traveledby the leader. A spacing control system operatively associated with thepath tracking system and the follower vehicle causes the followervehicle to maintain a predetermined spacing between the follower vehicleand the leader.

Another embodiment of a vehicle control system for causing a followervehicle to follow a leader may comprise a tether mounted to the followervehicle, the tether having an end adapted to be attached to the leader.Length sensing means operatively associated with the tether senses alength of the tether extending between the follower vehicle and theleader. Angle sensing means operatively associated with the tethersenses an angle between the tether and the follower vehicle. A pathtracking system operatively associated with the length sensing means andthe angle sensing means determines a path traveled by the leader. A pathcontrol system operatively associated with the path tracking system andthe follower vehicle causes the follower vehicle to follow the pathtraveled by the leader. A spacing control system operatively associatedwith the path tracking system and the follower vehicle causes thefollower vehicle to maintain a predetermined spacing between thefollower vehicle and the leader.

Also disclosed is a method for causing a follower vehicle to follow aleader that comprises: Connecting the follower vehicle and the leaderwith a tether; measuring a length of the tether connecting the followervehicle and leader; measuring an angle between the tether and thefollower vehicle; determining a position of the leader based on themeasured length and the measured angle; determining a path traveled bythe leader based on a plurality of determined leader positions; steeringthe follower vehicle so that the follower vehicle substantially followsthe path traveled by the leader; and controlling a velocity of thefollower vehicle so that the follower vehicle maintains a predetermineddistance from the leader.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative and presently preferred exemplary embodiments of theinvention are shown in the drawings in which:

FIG. 1 is a pictorial representation of a leader/follower systemaccording to one embodiment of the present invention showing an inertialcoordinate system and a follower vehicle coordinate system;

FIG. 2 is a system block diagram of a leader/follower convoy systemillustrated in FIG. 1;

FIG. 3 is a pictorial representation of the follower vehicle controlsystem showing its relationship with various components of the followervehicle;

FIG. 4 is a pictorial representation, within the inertial coordinatesystem, of the leader and follower moving in a forward direction and ina reverse direction;

FIG. 5 is a perspective view of a tether system in accordance with anembodiment of the present invention;

FIG. 6 is a left side view in elevation of the tether system illustratedin FIG. 5;

FIG. 7A is a rear side view in elevation of an angle sensor of thetether system illustrated in FIGS. 5 and 6;

FIG. 7B is a cross-sectional view in elevation of the angle sensor takenalong the line B-B of FIG. 7A;

FIG. 8 is a schematic representation of a control loop involving thepath control system;

FIG. 9 is a schematic representation of the follower vehicle andlook-ahead point in the follower vehicle coordinate system;

FIG. 10 is a geometric illustration of the look-ahead point's relationto curvature used to control the follower;

FIG. 11 is a schematic representation of a control loop involving thevehicle spacing control system;

FIG. 12 is a pictorial representation, in the inertial coordinatesystem, of another embodiment for reverse movement of the leader andfollower vehicle; and

FIG. 13 is flow chart of a follower vehicle control method of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of a vehicle control system 1 for causing a followervehicle 4 to follow a leader 2 is best seen in FIGS. 1-5 and maycomprise a tether system 30 that is mounted to the follower vehicle 4.Tether system 30 includes a tether 6 having an end that is adapted to beattached to the leader 2, e.g., at point “B” of leader 2. A lengthsensor 36 (FIG. 5) operatively associated with tether system 30 senses alength l of tether 6 that extends between leader 2 and follower vehicle4 (e.g., between points “A” and “B”), as best seen in FIG. 1. Tethersystem 30 also includes an angle sensor 40 (FIG. 5) that senses an angleθ between tether 6 and the follower vehicle 4, as also best seen in FIG.1.

A path tracking system 10 (FIGS. 2 and 3) operatively associated withthe tether system 30 receives from the tether system 30 informationabout the length l of tether 6 and the angle θ that tether 6 makes withrespect to follower vehicle 4. Path tracking system 10 uses the tetherlength l and angle θ to determine a path 9 traveled by leader 2. A pathcontrol system 12 operatively associated with the path tracking system10 and the follower vehicle 4 causes the follower vehicle 4 to followthe path 9 traveled by leader 2. A spacing control system 14 operativelyassociated with the path tracking system 10 and the follower vehicle 4maintains a predetermined spacing or distance between the leader 2 andfollower vehicle 4.

Vehicle control system 1 may be operated to cause the follower vehicle 4to follow the path 9 traveled by leader 2. Consider, for example, asituation wherein the follower vehicle 4 has been connected to theleader 2 by the tether 6. As the leader 2 begins to move, it beginsdefining a path 9 that is to be followed by follower vehicle 4. The path9 is determined by measuring both the length l of tether 6 extendingbetween the leader 2 and follower vehicle 4 as well as by the angle θthat tether 6 makes with the follower vehicle 4. More specifically, eachmeasured length l and corresponding angle θ of tether 6 is used todefine a leader position or “traveled point” 8 in an inertial referenceframe X_(I), Y_(I), Z_(I), as illustrated in FIG. 1. A plurality ofleader positions or traveled points 8 thus define the path 9 traveled byleader 2. Consequently, as the leader 2 continues to move, defining path9 as it does so, the system 1 determines the path 9 traveled by theleader by measuring over time both the length l of tether 6 as well asthe angle θ that tether 6 makes with the follower vehicle 4.

In the embodiment shown and described herein, the path tracking systemreceives the length l and angle θ measurements from the tether system 30and produces or maps the path 9 traveled by the leader 2 in the inertialcoordinate system X_(I), Y_(I), Z_(I) illustrated in FIG. 1. The pathcontrol system 12 and spacing control system 14 receive from the pathtracking system 10 information (e.g., x,y coordinates in the inertialreference frame) relating to the path 9 traveled by follower vehicle 4.See FIG. 2. The path control system 12 steers the follower vehicle 4 sothat the follower vehicle 4 substantially follows the path 9 traveled bythe leader 2. Similarly, spacing control system 14 controls the speed orvelocity of the follower vehicle 4 so that the follower vehiclemaintains a predetermined distance from the leader 2.

A significant advantage of the present invention is that it provides acontrol system for a follower vehicle that does not require any input orcommunication from the leader 2. Follower vehicle 4 will simply beginfollowing leader 2 as soon as leader 2 begins to move. Consequently, thepresent invention is free of the problems and limitations associatedwith systems that require communication between the leader 2 andfollower vehicle 4.

Another advantage of the present invention is that it may be operated ineither the forward direction or the reverse direction. That is, inaddition to “leading” the follower vehicle 4, the leader 2 may be usedto back the follower vehicle 4 in the reverse direction. As will bedescribed in greater detail herein, operation of the system in thereverse direction may be accomplished in accordance with a variety ofmodes. For example, in one reverse operation mode, the leader 2 may backthe follower vehicle 4 in a manner akin to backing a trailer. In anothermode, the follower vehicle 4 may be backed along the same path 9originally defined by leader 2.

Still another advantage of the present invention is that it does notrequire the follower vehicle to have a driver or operator. Indeed, inthe embodiments shown and described herein, the follower vehicle 4 maybe completely autonomous. In one autonomous embodiment, the controlsystem 1 is provided with a direction control system 16 (FIG. 3). Thedirection control system 16 may be used to automatically select a traveldirection for the follower vehicle 4 (e.g., by operating thetransmission of the follower vehicle 4 in either the forward (i.e.,“Drive”) or reverse modes) by sensing (e.g., via tether 6) whether theleader is beginning to move in the forward direction or the reversedirection. Alternatively, such autonomous operation is not required, andfollower vehicle 4 may require the presence of an operator to perform atleast some operations (e.g., starting the engine of the vehicle,selecting the appropriate transmission gear, etc.).

Still yet other advantages are associated with the vehicle controlsystem of the present invention. For example, because the presentinvention does not require the leader 2 to provide any propulsive ortowing force to the follower vehicle 4, any of a wide range of leaders 2and follower vehicles 4 may be used without regard to type or sizedifferential between leader 2 and follower vehicle 4. For example, inone embodiment, the leader 2 may comprise a small automobile or pick-uptruck, whereas the follower vehicle 4 may comprise a heavy truck havingthe capacity to haul a large payload. In another embodiment, the leader2 may comprise a much smaller vehicle, such as a motorcycle, or even abicycle. Indeed, leader 2 need not even comprise a vehicle at all, andcould in another embodiment, comprise a person walking on foot.

Still yet another advantage of the control system 1 of the presentinvention is that it is not limited to use with convoys comprising onlyone leader 2 and one follower vehicle 4. For example, in anotherembodiment additional follower vehicles (with corresponding controlsystems 1) could be attached to follower vehicle 4 in a sequentialmanner. In such a configuration, each additional follower vehicle wouldbecome the leader for the vehicle immediately behind.

Having briefly described the vehicle control system 1 according to oneembodiment of the present invention, as well as some of its moresignificant features and advantages, various embodiments and alternativeconfigurations of the vehicle control system 1 and methods for causing afollower vehicle to follow a leader will now be described in detail.

Referring back now to FIGS. 1-4, one embodiment of a vehicle controlsystem 1 is shown and described herein as it could be utilized on asingle follower vehicle 4 to follow a single leader 2, although otherconvoy configurations are possible. In the embodiment shown anddescribed herein, both the leader 2 and the follower vehicle 4 comprisetrucks. However, other types of vehicles could be used. For example, andas mentioned above, the leader 2 need not even comprise a vehicle, butcould instead comprise any of a wide variety of “moving systems,” suchas bicycles, motorcycles, or even pedestrians. Similarly, followervehicle 4 may comprise any of a wide range of vehicles. However, it isgenerally desired, but not required, that follower vehicle comprise avehicle that could be provided with systems to allow it to beautonomously operated (i.e., without driver input or supervision).Alternatively, however, the follower vehicle 4 need not be fullyautonomous and may require some degree of driver input. Still othervariations are possible, as would become apparent to persons havingordinary skill in the art after having become familiar with theteachings provided herein. Consequently, the present invention shouldnot be regarded as limited to any particular type of leader 2 orfollower vehicle 4.

The follower vehicle 4 is provided with a tether 6, one end of which isconfigured to be attached to the leader 2. Accordingly, tether 6 willextend between point “A” on follower vehicle 4 and point “B” on leader2, as best seen in FIG. 1. In the embodiment shown and described herein,tether 6 may comprise a portion of a tether system 30 (FIGS. 2 and 3)having a length sensor 36 and an angle sensor 40 (both of which areillustrated in FIG. 4). Length sensor 36 allows tether system 30 tomeasure the length l of tether 6 between point “A” on follower vehicle 4and point “B” on leader 2. Angle sensor 40 measures the angle θ thattether 6 makes with the follower vehicle 4 (e.g., the X_(b) axis offollower vehicle 4). As will be described in much greater detail herein,the control system 1 utilizes the length l and angle θ of tether 6 todetermine the path 9 traveled by leader 2 in inertial coordinate systemor frame, X_(I), Y_(I), Z_(I), depicted in FIG. 1. The system alsoutilizes a body coordinate system or frame, X_(b), Y_(b), that is fixedwith respect to the follower vehicle 4, as also depicted in FIG. 1.

Referring primarily now to FIGS. 2 and 3, the control system 1 may alsocomprise a path tracking system 10. Path tracking system 10 receivesdata from the tether system 30 (e.g., the measured values of the lengthl and angle θ of tether 6) and uses that information to determine forleader 2 a “leader position” or “traveled” point 8 (FIGS. 1 and 4). Theleader position or traveled point 8 may be represented as x,ycoordinates in the inertial coordinate system or frame X_(I), Y_(I),Z_(I), depicted in FIG. 1. Path tracking system 10 utilizes a pluralityof tether measurements (e.g., length l and angle θ) taken over time toidentify a plurality of traveled points 8 which, taken together, definepath 9 traveled by leader 2.

System 1 may also be provided with a path control system 12 that isoperatively associated with the path tracking system 10. Path controlsystem 12 receives the position data from the path tracking system 10and uses that data to produce a desired curvature “k” to be followed bythe follower vehicle 4 in the manner that will be described in greaterdetail below. A spacing control system 14 receives length or spacingdata and uses that data to produce a desired velocity “V” for thefollower vehicle 4. As will also be described in much greater detailbelow, the desired velocity “V” is used to maintain a predeterminedspacing between the follower vehicle 4 and leader 2.

System 1 may also comprise a dead reckoning system 11 that isoperatively associated with the vehicle 4. As its name implies, deadreckoning system 11 provides estimates of the position (e.g., x, y) ofthe follower vehicle 4 as well as its heading “φ”. The estimates of theposition and heading of follower vehicle 4 are used in the mannerdescribed below. Dead reckoning system 11 may comprise any of a widerange of systems and devices suitable for providing estimates of theposition and heading of follower vehicle 4. By way of example, in oneembodiment, dead reckoning system 11 may comprise an odometer andsteering angle sensor (not shown). Alternatively, an odometer and aheading gyroscope could also be used. Still other variations arepossible. For example, in yet another embodiment, the dead reckoningsystem 11 could comprise an inertial platform (e.g., comprising eithermechanical gyros or laser gyros). In still yet another embodiment,position information could be provided by a global positioning system(GPS) receiver.

However, because dead reckoning systems of the type that could beutilized herein are well known in the art and could be readily providedby persons having ordinary skill in the art after having become familiarwith the teachings provided herein, and because the details of the deadreckoning system are not required to understand the present invention,the particular dead reckoning system 11 that may be utilized in variousembodiments will not be described in further detail herein.

As mentioned above, it is generally preferred, but not required, thatthe follower vehicle 4 be fully autonomous, i.e., so that it can operateand follow the leader 2 without requiring driver input. In such anembodiment, follower vehicle 4 may need to be provided with additionalsystems and devices to allow the follower vehicle 4 to respond to thevehicle control system 1 in a way that will allow the follower vehicle 4to operate fully autonomously. For example, and with reference nowprimarily to FIG. 3, follower vehicle 4 may be provided with apropulsion system 18, a braking system 20, and a steering system 22,each of which is operatively connected to a vehicle control unit (VCU)24. Vehicle control unit 24 may be provided with a steering controlsystem 26 and a velocity control system 28 to allow the vehicle controlunit 24 to operate the propulsion system 18, braking system 20, andsteering system 22 of follower vehicle 4. The vehicle control unit 24 isalso operatively associated with the vehicle control system 1 and isresponsive to control signals produced thereby. Stated simply, then, inthe embodiment illustrated in FIG. 3, the vehicle control unit 24 actsas the interface between the various mechanical systems of followervehicle 4 (e.g., the propulsion system 18, braking system 20, andsteering system 22) and the vehicle control system 1.

Alternatively, other vehicle control configurations are possible, aswould become apparent to persons having ordinary skill in the art afterhaving become familiar with the teachings provided herein. Consequently,the present invention should not be regarded as limited to anyparticular configuration for interfacing with the various mechanicalsystems of follower vehicle 4.

Referring now to FIGS. 5, 6, 7A, and 7B, the tether system 30 of oneembodiment may comprise a spool 34 sized to receive a length of tether6. Tether 6 may comprise any of a wide range of generally lightweight,cord-like materials, preferably having high tensile strengths. By way ofexample, in one embodiment, tether 6 comprises an aeromatic copolyamidmaterial available from Teijin Limited, of Tokyo Japan under thetrademark “Technora”. Alternatively other materials may also be used.

Spool 34 is mounted for rotation within tether system 30, e.g., viaspool shaft 34 a, and is driven by a motor 32 via drive belt 33 andsprocket or sheave 34 b. A length sensor 36 operatively associated withthe spool 34 may be used to measure the amount or length of tether 6that is unwound from spool 34. By way of example, in one embodiment,length sensor 36 comprises a multiple turn absolute encoder, althoughother sensors could be used as well.

In operation, motor 32 pays out or reels in the tether 6 as necessary,depending on the positions and velocities of leader 2 and follower 4, aswill be described in greater detail below. In addition, motor 32 is usedto keep tether 6 under a predetermined tension, e.g., by applying aconstant torque to spool 34. For example, if the tension in tether 6exceeds the desired amount (e.g., as a result of the leader 2accelerating), motor 32 may be controlled or operated to pay out thetether 6 until the desired torque is re-attained and the predeterminedtension is once again present in tether 6. The predetermined tension intether 6 is such that there is a certain amount of sag allowed in thelength of tether 6 extending between the follower vehicle 4 and leader2. The tension may be varied depending on the distance between theleader 2 and follower vehicle 4 to ensure that tether 6 does not drag onthe ground. The tension in tether 6 is also important in obtainingaccurate measurements of the angle θ.

Tether system 30 may also be provided with a tether guide 38 to evenlydistribute the tether 6 onto spool 34 as spool 34 reels in tether 6. Asspool 34 rotates, the tether guide 38 slides axially along a rotatingshaft 38 a which is rotated via sprocket or sheave 38 b and fixed guideshaft 38 c. The rotating shaft 38 a and guide shaft 38 c run parallel toeach other and to the spool shaft 34 a. The tether guide 38 furtherincludes a transmission 38 d that translates the rotation of therotating shaft 38 a into axial movement along the rotating shaft 38 aand guide shaft 38 c. Thus, the rotational direction of the rotatingshaft 38 a dictates the axial direction of the tether guide 38. As thespool 34 rotates, the tether 6 slides past the guide roller 38 e of thetether guide 38 while the tether guide 38 moves axially to evenlydistribute the tether 6.

Referring to FIGS. 5 and 6, a belt 33 links the motor 32, spool 34 andtether guide 38. The belt 33 wraps around a shaft 32 a of the motor 32and passes over the sprockets or sheaves 34 b and 38 b. Thus, the belt33 engages the shaft 32 a, and pulleys 34 b, 38 b to deliver the outputof the motor 32 to the spool 34 and tether guide 38. That is, therotation of the shaft 32 a rotates the spool shaft 34 a (via pulley 34b) and the rotating shaft 38 a (via pulley 38 b). Accordingly, therotational direction of the shaft 32 a dictates the rotational directionof the spool 34 and the axial direction of the tether guide 34.

The angle sensor 40 measures the angle θ between the tether 6 and thedirection of travel (X_(b)-axis) of the follower 4, as illustrated inFIG. 1. Referring now primarily to FIGS. 5-7B, the angle sensor 40 ismounted to and extends outwardly from the tether guide 38. The tether 6slides through the angle sensor 40 as the spool 34 rotates. The anglesensor 40 includes a sensor 42, a mounting bracket 44, a base 46, a baseshaft 48, first and second pairs of rollers 50, 52, and an arm 54.Sensor 42 senses or measures the rotation of arm 54 and, in oneembodiment, may comprise a single-turn, absolute, 13-bit opticalencoder. Alternatively, other types of sensors could be used as well.

Referring to FIGS. 7A and 7B, the angle sensor 40 is mounted to thetether guide 38 by the mounting bracket 44. The base 46 extendsoutwardly from the mounting bracket 44 and ends with a vertical baseshaft 48. The sensor 42 is connected to the base shaft 48 and the arm 54is rotatably mounted to the base shaft 48 with a bearing 64 disposedtherebetween. The arm 54 rotates about the base shaft 48 as the angle θchanges. That is, as the leader 2 causes the tether 6 to move, the arm54 also moves. This movement of the arm 54 is sensed by sensor 42 andtranslated to provide a measurement of the angle θ. Bolts 62 attach apair of first rollers 50 to an under side of an outward end of the arm54. Bolts 62 also attach a pair of second rollers 52 to an under side ofthe base 46. The first and second rollers 50, 52 are disposed on theangle sensor 40 to allow the tether 6 to slide through the angle sensor40 yet cause appropriate movement of the arm 54 to convey the angle θ ofthe tether 6.

Angle sensor 40 may also be provided with a damper 58 that is coupled tothe second rollers 52 and an end of a swing arm 60. The other end of theswing arm 60 is attached to a bolt 62 secured to the under side of alower platform 56. The lower platform 56 is coupled to the first rollers50. The damper 58 is preferably a rotary damper and damps out theoscillations in the tether 6 as both the leader 2 and follower 4 aremoving.

The present invention advantageously provides a follower vehicle controlsystem 1 and method 100 that causes the follower 4 to follow the generalpath of the leader 2 while maintaining a predetermined spacing betweenthe leader 2 and the follower 4. In order to follow the general path ofthe leader 2, the path tracking system 10 uses the tether measurements(angle θ and length l) obtained from the tether unit 30. The pathtracking system 10 uses the angle θ and the length l to calculate theposition of the leader 2 with respect to the follower 4. Once the pathtracking system 10 determines the path that the leader 2 has traveled,the path control system 12 and the vehicle spacing control system 14control the follower 4 via the VCU 24 (FIG. 3) to generally follow thesame path while maintaining the correct spacing between the follower 4and the leader 2.

Referring to FIGS. 1 and 3, the path tracking system 10 is configured todetermine a plurality of traveled points 8 in a path traveled by theleader 2 based on data from the tether unit 30. The path tracking system10 uses two different coordinate frames in conjunction to track theleader 2, as shown in FIG. 1. The first is the inertial coordinatesystem or frame (O_(I) X_(I) Y_(I)) and the second is the follower bodycoordinate system or frame (O_(b) X_(b) Y_(b)). The follower body frame(O_(b) X_(b) Y_(b)) is fixed on the follower 4 such that the originO_(b) is located at the control point of the follower 4 (e.g. midpointof the rear axle) and the X_(b)-axis is in the direction of travel ofthe follower 4. The angle θ is measured from the X_(b)-axis of thefollower body frame (O_(b) X_(b) Y_(b)) to the tether 6. The heading ofthe follower 4 is the angle φ and is measured from the inertialcoordinate frame X_(I)-axis to the follower body frame X_(b)-axis.

The inertial coordinate frame (O_(I) X_(I) Y_(I)) is fixed in space andmay be the same as the follower body frame (O_(b) X_(b) Y_(b)) atinitialization (e.g. at power-on). Still referring to FIG. 1, at aninitial stage when the leader 2 begins movement, traveled points 8 haveyet to be acquired. During this brief initial stage, the path trackingsystem 10 determines a straight line l_(s) drawn from the control pointO_(b) to the point where the tether 6 attaches to the leader 2 (point B)using the distance between the control point O_(b), the angle θ and thelength l of the tether 6. The path tracking system 10 then selectspoints along the line l_(s) and stores them as an inertial coordinate(x_(I), y_(I)) in memory 13 until traveled points 8 begin to be obtainedand stored in memory 13. Thus, the path control system 12 controls thefollower 4 to the selected points along the line l_(s) stored in memory13 and then transitions to controlling the follower 4 to traveled points8 stored in memory 13.

From the measured angle θ and length l of the tether 6, the x and ycoordinate position of the point where the tether 6 attaches to theleader 2 (i.e., point “B”) may be determined in the follower body frame(O_(b) X_(b) Y_(b)). However, in order for the point “B” to be stored asa traveled point 8 traveled by the leader 2, the path tracking system 10transforms the point B into the inertial coordinate frame (O_(I) X_(I)Y_(I)). After each point “B” that is sampled is transformed into theinertial coordinate frame (O_(I) X_(I) Y_(I)), the path tracking system10 stores the transformed point “B” in memory 13 as traveled point 8. Anarray or plurality of traveled points 8 defines the path 9 taken by theleader 2 (a measured path). In one embodiment, once a traveled point 8is reached by the follower 4 it is erased from storage. Alternatively, aset number of traveled points 8 are stored (e.g. a sufficient number oftraveled points 8 to travel 100 meters) regardless of whether thefollower 4 has already reached the traveled point 8. Updated traveledpoints 8 replace the least recently traveled points 8 in storage, i.e.first in first out. The storage of a set number of traveled points 8 ismost advantageous when the leader 2 reverses movement to reverse theconvoy, as discussed below.

From the measurements of angle θ and length l, the x and y coordinateposition of the leader in the body frame (O_(b) X_(b) Y_(b)) is

$B^{b} = \begin{bmatrix}{d_{t} + {l\; \cos \; \varphi}} \\{l\; \sin \; \varphi}\end{bmatrix}$

where the superscript b denotes that the point “B” is with respect tothe body frame and d_(t) is the distance from the point O_(b) to thetether mounting location which lies directly on the x-axis of thefollower body frame (O_(b) X_(b) Y_(b)).

The position B^(I) of the leader 2 in the inertial coordinate frame(O_(I) X_(I) Y_(I)) is calculated using a homogeneous transformation

$B^{I} = {H_{b}^{I}\begin{bmatrix}B_{2 \times 1}^{b} \\1_{1 \times 1}\end{bmatrix}}$

where H^(I) _(b) is the homogeneous transformation matrix and where thesuperscript I is the reference frame to which the point will betranslated and the subscript b is the current reference frame of thepoint that is being transformed. The homogeneous transformation matrixis

$H_{b}^{I} = \begin{bmatrix}R_{b{({2 \times 2})}}^{I} & O_{b{({2 \times 1})}}^{I} \\O_{({1 \times 2})} & 1_{({1 \times 1})}\end{bmatrix}$

where R^(I) _(b) is the rotation matrix from the follower body frame tothe inertial frame and the point O^(I) _(b) is the location of thefollower 4 in the inertial frame. The rotation matrix used in Equation2.3 is

$R_{b}^{I} = \begin{bmatrix}{\cos \; \varphi} & {{- \sin}\; \varphi} \\{\sin \; \varphi} & {\cos \; \varphi}\end{bmatrix}$

The position of the follower (O_(b) in FIG. 1) is critical to theperformance of the entire system. The ability of the follower vehiclecontrol system 1 and method 100 of the present invention to control thefollower 4 to drive the path of the leader 2 depends on the determinedlocation O_(b) of the follower 4 itself. The position of the follower 4(control point O_(b)) is calculated using dead-reckoning (e.g., by deadreckoning system 11) by the path tracking system 10. The dead-reckoningsystem 11 calculates the current position of the follower 4 bycalculating a Δx and a Δy that the follower 4 has traveled in one sampletime and adding that to the last calculated position. The equations are

${\Delta \; x} = {\frac{\Delta \; \beta \; R}{\lambda}{\cos (\varphi)}}$${\Delta \; y} = {\frac{\Delta \; \beta \; R}{\lambda}{\sin (\varphi)}}$

where φ is the heading of the follower 4, Δβ is the change in angle ofan output shaft (not shown) that comes off the front wheel differentialof the follower 4, λ is the gear ratio from the output shaft to the ringgear (not shown) in the differential and R is the radius of thefollower's 4 front wheels. The value

$\frac{\Delta \; \beta}{\lambda}$

gives an average rotation angle of the two front wheels. The heading φmay be measured using a fiber-optic gyro and the change in wheelrotation angle may be measured using an incremental optical encoder.Preferably, the dead-reckoning system 11 calculates the position O_(b)within ±0.8 m after 50 m of driving, where 50 m is the maximum length ofthe tether 6, which is the maximum distance that a known point can befrom the current position of the follower 4.

Because the path tracking system 10 measures the position of the leader2 with respect to the follower 4 and then stores the point withreference to the inertial coordinate frame (O_(I) X_(I) Y_(I)), themaximum amount of dead-reckoning with respect to a sampled point shouldbe 50 m.

The actual position of the leader 2 in the inertial coordinate frame(O_(I) X_(I) Y_(I)) is offset by the same amount of error in theestimate of the true position of the follower 4 in the inertialcoordinate frame (O_(I) X_(I) Y_(I)) at the time when the measurementfrom the tether 6 is taken. That is, when a measurement is taken, thereis no relative error from dead-reckoning between the leader 2 and thefollower 4 in the inertial coordinate frame (O_(I) X_(I) Y_(I)) whichmeans that dead-reckoning is needed only as far as required to make itto the traveled point 8.

Path Control System

Once the path tracking system 10 determines the path that the leader 2has traveled, the path control system 12 and the vehicle spacing controlsystem 14 control the follower 4 via the VCU 24 to generally follow thesame path as the leader 2 while maintaining the correct spacing betweenthe follower 4 and the leader 2.

FIG. 8 shows the interaction of the path control system 12 with thesteering control system 26, dead reckoning system 11 and the followervehicle 4. The velocity V shown as an input to the follower vehicle 4 inFIG. 8 is commanded by the vehicle spacing control system 14 (FIG. 2)and is discussed below. The input into the path control system 12 of thefollower vehicle control system 1 is the error calculated from theposition (x_(p), y_(p)) where the follower 4 should be (i.e., a selectedtraveled point 8) versus the actual position (x_(a), y_(a)) of thefollower 4. Specifically, the error is calculated from the value of thecoordinates (x_(p), y_(p)) that the follower 4 should reach, i.e., thecoordinates of the selected traveled point 8, subtracted from the actuallocation (x_(a), y_(a)) of the follower 4 (determined via dead-reckoningsystem 11). The error is passed into the path control system 12. In oneembodiment, path control system 12 utilizes a control algorithm known asa “Pure Pursuit” algorithm developed by Carnegie Mellon University anddescribed in “Implementation of the Pure Pursuit Path TrackingAlgorithm,” R. C. Coulter, Tech. rep., Carnegie Mellon University, 1992,which is incorporated herein by reference for all that it discloses.Alternatively, other types of control algorithms that are now known inthe art or that may be developed in the future may be utilized by pathcontrol system 12.

The path control system 12 uses the Pure Pursuit algorithm to calculatea desired curvature k_(d) that will drive the follower 4 to the correctlocation (traveled point 8). The curvature k_(d) of the follower 4 isthe inverse of the turn radius of the follower 4. The turn radius of thefollower 4 refers to the radius of the control point of the follower 4.In this embodiment, the control point is the origin O_(b) of thefollower body frame (O_(b) X_(b) Y_(b)) in the middle of the two reartires. The desired curvature k_(d) is then passed to the steeringcontrol system 26, which controls the steering angle of the follower'swheels such that the desired curvature k_(d) is achieved. Because oferrors in measurements and other small errors the steering controlsystem 26 does not output the precise desired curvature but outputs aslightly different curvature which is the actual curvature k_(a). It isthis actual curvature k_(a) that causes the follower's 4 positioncoordinates to change. After the follower 4 has moved in response to theinputs of desired velocity V_(d) and actual curvature k_(a), the actualfollower's position coordinates are estimated by the dead-reckoningsystem 11 and then used to calculate the new error e in the follower'sposition.

Referring to FIGS. 8-10, the details of how the follower 4 is controlledto correct the error discussed above i.e., maneuver to the path of theleader 2 with the selected traveled point 8 as the goal, will now bediscussed. The Pure Pursuit algorithm utilized by the path controlsystem 12 uses a look-ahead point (x_(La), y_(La)) to calculate acurvature k_(d), that will drive the follower 4 back onto the path takenby the leader 2 and thereby correct the error e. The error e will changedue to inputs of curvature k_(d). A look-ahead vector L_(a) is used todesignate the look-ahead point (x_(La), y_(La)). The look-ahead vectorL_(a) extends from the control point O_(b) toward the path traveled bythe leader 2. The look-ahead point (x_(La), y_(La)) is located at theintersection of the look-ahead vector L_(a) and the path of the leader2. A look-ahead distance l_(La) is the distance from the look-aheadpoint (x_(La), y_(La)) to the control point O_(b) of the follower 4along the look-ahead vector L_(a).

Still referring to FIGS. 8-10, in determining the look-ahead point(x_(La), y_(La)), a path coordinate frame (O_(p) X_(p) Y_(p)) is placedtangent to the path of the leader 2 and aligned with the follower 4 suchthat the x position of the control point O_(b) in the path coordinateframe (O_(p) X_(p) Y_(p)) is zero. The Y_(p)-axis points toward thecenter of the leader's 2 traveled “circular” path. That is, the pathtraveled by the leader 2 is described in terms of a varying radiusr_(p), noting that as the path approaches a straight line, r_(p)approaches infinity. The angle φ is the heading of the follower 4 aspreviously described. The angle α is the angle between the X_(b)-axis ofthe body frame and the look-ahead vector L_(a). The angle β is definedas the angle between the look-ahead point (x_(La), y_(La)) and theX_(p)-axis of the path coordinate frame (O_(p) X_(p) Y_(p)). The errorinputted into the path control system 12 can be

e=l _(La) sin(α).

Furthermore, the y coordinate y_(La) of the look-ahead point (x_(La),y_(La)) and the y coordinate of the follower 4 in the path coordinateframe (O_(p) X_(p) Y_(p)) can be expressed as

l _(La) sin(β)=y _(La) −y.

In addition, y_(La) can be expressed as

y _(La) =r _(p) −r _(p) cos(θ)

where θ is the angle through which the look-ahead point (x_(La), y_(La))has gone around the path ahead of the vehicle.

As the look-ahead point (x_(La), y_(La)) moves along the path, y_(La)increases. This increase will cause a change in the error e.Accordingly, the desired error into the path control system 12 can bebased on the current path of the leader 2 with the desired errorapproaching zero as the path of the leader 2 approaches a straight line.The desired error will increase as the radius of the turn decreases andas the velocity increases. In driving, typically as the turn radiusdecreases, the velocity must decrease in order to make the turn withoutsliding.

Referring primarily to FIGS. 9 and 10, the look-ahead point isillustrated in terms of the body frame (O_(b) X_(b) Y_(b)). The pathcontrol system 12 uses the look-ahead distance l_(La) that is used asthe radius of a circle that surrounds the follower, with the originbeing at O_(b). More specifically, the circle connects the look-aheadpoint (x_(La), y_(La)) and the control point O_(b), while being tangentto the X_(b)-axis;

$\frac{1}{k_{d}}$

is the radius of the circle. The desired curvature k_(d) can be solvedfor using the Pythagorean theorem. The y coordinate value where the

$\frac{1}{k_{d}}$

radius meets the Y_(b)-axis is

$\frac{1}{k_{d}} = {y_{La} + d}$

Using the Pythagorean theorem with

$\frac{1}{k_{d}}$

as the hypotenuse and solving for k_(d) gives

$k_{d} = {\frac{2\; y_{La}}{l_{La}^{2}}.}$

Thus, the desired curvature k_(d) commanded or outputted by the pathcontrol system 12 can be geometrically calculated such that if thefollower 4 were to drive the desired curvature k_(d) (adjusted inapplication to the actual curvature k_(a)) the follower 4 would arriveat the look-ahead point (x_(La), y_(La)). The proportional gain is

$\frac{2}{l_{La}^{2}}$

and the gain changes as the look-ahead distance changes. Thus, the errore comes from the look-ahead point (x_(La), y_(La)) and not from thecontrol point O_(b) of the follower 4. The look-ahead point (x_(La),y_(La)) acts as a predictor for the error of the follower 4 because thelook-ahead point (x_(La), y_(La)) is forward of the follower 4.

The steering control system 26 controls the follower 4 to a desiredcurvature k_(d) by outputting an actual curvature k_(a). The steering ofthe follower 4 is operated by the steering system 22. In one embodiment,the steering system includes a DC motor (not shown) connected to thesteering shaft (not shown) of the follower vehicle 4. The DC motorrotates the steering shaft, which is coupled to a power steering gearbox(not shown) that assists in turning the wheels (adjusting the angle ofthe wheels). An encoder (not shown) mounted to the shaft of the motormeasures the angular position of the shaft for feedback to the steeringcontrol system 26. The angular position of the motor shaft correspondsto the angle of the wheels of the follower 4. Thus, the angular positionof the motor shaft is mapped into curvature k_(d). That is, the steeringcontrol system 26 is fed a desired curvature k_(d) and controls the DCmotor to the desired curvature k_(d) by adjusting the angular positionof the motor shaft that produces the actual curvature k_(a).

As the follower vehicle control system 1 and method 100 controls thefollower 4 to stay on the same general path as the leader 2, thefollower vehicle control system 1 and method 100 also controls thedistance between the follower 4 and the leader 2. In order to ensurethat the follower 4 and the leader are a safe distance apart at alltimes, the vehicle spacing control system 14 commands a velocity thatproduces the desired distance d_(d) between the leader 2 and thefollower 4. The length d_(d) is a predetermined value that isproportional to the velocity of the leader 2 and/or follower 4. Thevehicle spacing control system 14 commands the VCU 24 to adjust thepropulsion system 18 and/or the braking system 20 of the follower 4 tomaintain the predetermined, desired distance d_(d).

FIG. 11 illustrates how the vehicle spacing control system 14 interactswith the follower 4. The input to the vehicle spacing control system 14is the vehicle spacing distance error e_(d). This error is

e _(d) =d _(d) −d _(a)

where d_(d) is the desired distance and d_(a) is the actual distance ofthe path between the follower 4 and the leader 2. That is, the actualdistance d_(a) is the length between the leader 2 and the follower 4along the path outlined by the traveled points 8. The vehicle spacingcontrol system 14 communicates with the path tracking system 10 toobtain data for traveled points 8 and the location of the control pointO_(b). The actual distance d_(a) is measured by summing the magnitude ofeach distance between the traveled points 8 for all of the acquiredtraveled points 8 not yet reached by the follower 4.

The follower vehicle control system 1 and method 100 of the presentinvention is advantageously set up to accommodate a leader 2 that is nota vehicle at all. FIG. 11 illustrates the arrangement where the positionof the leader 2 is a disturbance in the control loop. The vehiclespacing control system 14 is designed to reject the disturbance input.

The distance between the leader 2 and follower 4 is controlled by theposition of the follower 4 with respect to the leader 2. A velocitycontrol system 28 of the VCU 24 that adjusts a throttle (not shown) inthe propulsion system 18 and/or a brake actuator (not shown) in thebraking system 20 controls the velocity.

Referring back now primarily to FIG. 3, the direction control system 16of the follower vehicle control system 1 communicates with the tetherunit 30 and the VCU 24 and commands the VCU 24 to shift into a desiredgear. Thus, the general movement selection (e.g. Park, Reverse, Forward)of the follower 4 is not user-selected but rather, is determined by thefollower vehicle control system 1 and method 100 of the presentinvention.

The direction control system 16 selects the direction of the followerbased on the direction of rotation of the spool 34. Specifically, thedirection control system 16 requests and receives information from thetether unit 30. As described above, the motor 32 and spool 34 areconfigured to maintain a predetermined amount of tension by eitherretracting the tether 6 when tension falls or paying-out the tether 6when tension rises. The direction control system 16 can also receiveinformation from the VCU 24 or sensors (not shown) to determine whetherthe follower 4 has ceased movement. The direction control system 16 isprogrammed to detect when the movement of the follower has ceased formore than a predetermined amount of time (e.g., 3 seconds). After thepredetermined amount of time, the direction control system 16 monitorsthe direction of rotation of the spool 34. This monitoring of the spool34 essentially detects the movement of the leader 2. That is, if thedirection control system 16 detects rotation of the spool 34 such thatthe tether 6 is being payed-out, then the leader 2 is moving forward.If, on the other hand, the direction control system 16 detects rotationof the spool 34 such that the tether 6 is being reeled-in, then theleader 2 is moving in reverse. After the predetermined amount of timewhere movement has ceased, the direction control system 16 selects agear according to the rotation of the spool 34. Thus, if the leader 2reverses, the direction control system 16 senses the according rotationof the spool 34 and shifts the follower 4 into reverse. Likewise, if thedirection control system 16 senses the rotation of the spool 34identified with forward movement of the leader 2, the direction controlsystem shifts the follower 4 into drive. The shift into drive or reverseis accomplished by a corresponding command from the direction controlsystem 16 to the VCU 24, which then implements the commanded shifting.

One of the advantages of the present invention is that in order toreverse the follower 4, a leader 2 need only stop for a predeterminedamount of time and reverse itself to cause a decrease in tension in thetether 6. As the leader 2 reverses, the follower 4 also reverses.Referring to FIGS. 4 and 12, two modes for reverse movement aredescribed herein. In the first mode for reverse movement, shown in FIG.12, the path control system 12 commands the steering control system 60of the VCU 24 to turn the turning wheels of the follower 4 at an angleδ, which is proportional to the measured angle θ. In the secondembodiment, depicted in FIG. 4, the traveled points 8 that werepreviously reached by the follower 4 are once again set as goals for thefollower 4 to reach (this time in reverse).

The first mode will now be described in more detail. FIG. 12 shows thesame angles θ, φ, X_(b)-axis and X_(I)-axis as FIG. 1 and alsoillustrates angle δ measured from a center line of the tire to theX_(b)-axis. Just as the angle θ of the tether 6 from the follower 4 tothe leader 2 is measured when moving forward, so too is the angle θmeasured during reverse movement. In addition, the steering controlsystem 60, which receives a command for the angle δ from the pathcontrol system 12, controls the steering system 30 of the follower 4 toproduce the angle δ at the turning wheels. The angle δ is based on anddetermined from the angle θ of the tether 6. That is, the angle δ isproportional to the angle θ of the tether 6. With the self-retractingspool 34, there is enough tension in the tether 6 to measure the angle θand thus, sufficient data to use for controlling the follower's wheelssuch that they turn at angle δ. In this way, the present inventioncauses the follower 4 to act like a trailer hitched to the leader 2.That is, the backwards movement of the follower 4 mimics a trailerattached to the leader 2.

In an alternative reverse or backing mode, a number of the traveledpoints 8 (e.g., corresponding to 100 meters) that the follower 4 hasdriven past are stored by the path tracking system 10 in memory 13. Thepath control system 12 uses these stored traveled points 8 duringreverse movement to control the follower 4. Once the direction controlsystem 16 commands a shift into reverse, the most recently attainedtraveled point 8 is sent from the path tracking system 10 to the pathcontrol system 12. As is done for forward movement, the path controlsystem 12, as well as the vehicle spacing control system 14, make thenecessary calculations and instruct the VCU 24 to control the propulsionsystem 18, braking system 20 and steering system 22 of the follower 4 tofollow the previously traveled path outlined by the traveled points 8 inreverse—while maintaining the correct spacing between the follower 4 andthe leader 2.

The follower vehicle control method 100 of the present inventioncontrols the follower 4 attached to the leader 2 by the tether 6.Referring to FIG. 13, the follower vehicle control method 100 includesmeasuring an angle θ between a heading of the follower 4 and the tether6 and measuring a length l of the tether 6 between the follower 4 andthe leader 2 (S102). A path of the leader 2 is tracked by determining aplurality of traveled points 8 in a path of the leader based on theangle θ and the length l (S104). The follower vehicle control method 100further includes controlling the path of the follower 4 by driving thefollower 4 to one or more of the traveled points 8 (S106). The velocityof the follower 4 is control system to maintain a predetermined spacingbetween the leader 2 and the follower 4 (S108).

In understanding the scope of the present invention, the term“comprising” and its derivatives, as used herein, are intended to beopen ended terms that specify the presence of the stated features,elements, components, groups, and/or steps, but do not exclude thepresence of other unstated features, elements, components, groups,and/or steps. The foregoing also applies to words having similarmeanings such as the terms, “including”, “having” and their derivatives.Any terms of degree such as “substantially”, “about” and “approximate”as used herein mean a reasonable amount of deviation of the modifiedterm such that the end result is not significantly changed. For example,these terms can be construed as including a deviation of at least ±5% ofthe modified term if this deviation would not negate the meaning of theword it modifies.

While only selected embodiments have been chosen to illustrate thepresent invention, it will be apparent to those skilled in the art fromthis disclosure that various changes and modifications can be madeherein without departing from the scope of the invention as defined inthe appended claims. For example, the size, shape, location ororientation of the various components can be changed as needed and/ordesired. Components that are shown directly connected or contacting eachother can have intermediate structures disposed between them. Thefunctions of one element can be performed by two, and vice versa. Thestructures and functions of one embodiment can be adapted to anotherembodiment. Any of the elements or units that perform data processingmay be implemented in software, firmware or hardware, or any suitablecombination thereof. It should be noted that while the present inventionis shown and described herein as it could be used in conjunction with aconfiguration of various hardware and software, it could be utilizedwith other configurations, either now known in the art or that may bedeveloped in the future, so long as the objects and features of theinvention are achieved, as would become apparent to persons havingordinary skill in the art after having become familiar with theteachings provided herein. Consequently, the present invention shouldnot be regarded as limited to that shown and described herein. It is notnecessary for all advantages to be present in a particular embodiment atthe same time. Thus, the foregoing descriptions of the embodimentsaccording to the present invention are provided for illustration only,and not for the purpose of limiting the invention as defined by theappended claims and their equivalents.

Having herein set forth preferred embodiments of the present invention,it is anticipated that suitable modifications can be made thereto whichwill nonetheless remain within the scope of the invention. The inventionshall therefore only be construed in accordance with the followingclaims:

1. A vehicle control system for causing a follower vehicle to follow aleader, comprising: a tether system operatively associated with a leaderand a follower vehicle; a path tracking system operatively associatedwith the tether system, the path tracking system determining a pathtraveled by the leader; a path control system operatively associatedwith the path tracking system, the path control system causing thefollower vehicle to travel to a traveled point on the path traveled bythe leader.
 2. The system of claim 1, further comprising a spacingcontrol system operatively associated with the tether system, leader,and follower vehicle, the spacing control system causing the followervehicle to maintain a predetermined distance between the followervehicle and the leader.
 3. The system of claim 1, wherein the tethersystem further comprises: a tether, the tether having a first endadapted to be attached to the leader and a second end adapted to beattached to the follower vehicle; a length sensor operatively associatedwith the tether, the length sensor sensing a length of the tetherextending between the follower vehicle and the leader; and an anglesensor operatively associated with the tether, the angle sensor sensingan angle between the tether and the follower vehicle.
 4. The system ofclaim 3, further comprising a direction control system operativelyassociated with the follower vehicle and the tether system, thedirection control system selecting between a forward travel directionand a reverse travel direction for the follower vehicle based on achange in length of the tether.
 5. The system of claim 1, wherein theleader is a moving system that is not a vehicle.
 6. The system of claim1, wherein the tether system is operatively associated with the leaderand the follower vehicle without propulsive or towing force between theleader and the follower vehicle.
 7. A vehicle control system for causinga follower vehicle to follow a leader, comprising: a tether systemmounted to the follower vehicle, the tether system comprising: a tetherhaving an end adapted to be attached to the leader; a length sensoroperatively associated with the tether, the length sensor sensing alength of the tether extending between the follower vehicle and theleader; and an angle sensor operatively associated with the tether, theangle sensor sensing an angle between the tether and the followervehicle; a path tracking system operatively associated with the tethersystem, the path tracking system determining a path traveled by theleader; a path control system operatively associated with the pathtracking system and the follower vehicle, the path control systemdetermining a determined path for the follower vehicle from an actualposition of the follower vehicle and a selected travel point on the pathtraveled by the leader, the path control system causing the followervehicle to follow the determined path for the follower vehicle; and aspacing control system operatively associated with the path trackingsystem and the follower vehicle, the spacing control system causing thefollower vehicle to maintain a predetermined spacing between thefollower vehicle and the leader.
 8. The system of claim 7, wherein thepath control system provides a desired curvature to the followervehicle, the desired curvature causing the follower vehicle to followthe path traveled by the leader.
 9. The system of claim 7, wherein thespacing control system causes the follower vehicle to maintain apredetermined spacing between the follower vehicle and the leader, thepredetermined spacing being measured along the path traveled by theleader.
 10. The system of claim 9, wherein the spacing control systemprovides desired velocity to the follower vehicle, the desired velocitycausing the follower vehicle to maintain the predetermined spacing. 11.The system of claim 7, further comprising a direction control systemoperatively associated with the follower vehicle and the tether system,the direction control system selecting between a forward traveldirection and a reverse travel direction for the follower vehicle basedon a change in length of the tether.
 12. The system of claim 7, furthercomprising a damper operatively associated with the angle sensor. 13.The system of claim 7, wherein the tether system further comprises: aspool mounted for rotation with respect to the tether system, the spoolholding a length of the tether; a motor operatively connected to thespool, the motor rotating the spool to pay-out and reel-in the tether;and a tether guide operatively associated with the motor, the spool, andthe tether.
 14. The system of claim 7, wherein the path control systemcausing the follower vehicle to follow the determined path for thefollower vehicle occurs automatically.
 15. A vehicle control system forcausing a follower vehicle to follow a leader, comprising: a tethermounted to the follower vehicle, the tether having an end adapted to beattached to the leader; a length sensing means operatively associatedwith the tether for sensing a length of the tether extending between thefollower vehicle and the leader; an angle sensing means operativelyassociated with the tether for sensing an angle between the tether andthe follower vehicle; a path tracking system operatively associated withthe length sensing means and the angle sensing means, the path trackingsystem determining a path traveled by the leader; a path control systemoperatively associated with the path tracking system and the followervehicle, the path control system determining a determined path for thefollower vehicle from an actual position of the follower vehicle and aselected travel point on the path traveled by the leader, the pathcontrol system causing the follower vehicle to follow the determinedpath for the follower vehicle; and a spacing control system operativelyassociated with the path tracking system and the follower vehicle, thespacing control system causing the follower vehicle to maintain apredetermined spacing between the follower vehicle and the leader. 16.The system of claim 15, further comprising a tension control meansoperatively associated with the tether for controlling a tension in thetether.
 17. The system of claim 16, wherein the tension control meansvaries the tension in the tether as a function of a velocity of theleader.
 18. A method for causing a follower vehicle to follow a leader,comprising: connecting the follower vehicle to the leader with a tether;measuring a length of the tether connecting the follower vehicle to theleader; measuring an angle between the tether and the follower vehicle;determining a position of the leader based on the measured length andthe measured angle; determining a path traveled by the leader based on aplurality of determined leader positions; determining a determined pathfor the follower vehicle from an actual position of the follower vehicleand a selected travel point on the path traveled by the leader; steeringthe follower vehicle so that the follower vehicle substantially followsthe determined path for the follower vehicle; and controlling a velocityof the follower vehicle so that the follower vehicle maintains apredetermined distance from the leader.
 19. The method of claim 18,wherein controlling a velocity of the follower vehicle comprisescontrolling a velocity of the follower vehicle so that the followervehicle maintains a predetermined distance from the leader, thepredetermined distance being measured along the path traveled by theleader.
 20. The method of claim 19, wherein the predetermined distanceis a function of the velocity of the leader.
 21. The method of claim 18,further comprising: operating the follower vehicle in a reverse traveldirection; and steering the follower vehicle in the reverse traveldirection to follow in reverse at least a portion of the path traveledby the leader.
 22. The method of claim 19, further comprising: operatingthe follower vehicle in a reverse travel direction; and steering thefollower vehicle in the reverse travel direction based on the measuredangle between the tether and the follower vehicle.
 23. The method ofclaim 19, further comprising selecting to operate the follower vehiclein either a forward travel direction or a reverse travel direction forthe follower vehicle based on a change in length of the tether.